Field of the Invention
The invention relates to a method for operating a vortex flowmeter device for measuring the flow of a fluid that flows through a measuring tube, with at least one baffle arranged in the measuring tube for producing eddies in the fluid, at least one first sensor and at least one second sensor for measuring the pressure fluctuations in the fluid that accompany the eddies, and with a signal-processing device for processing the signals x1 of the first sensor and the signals x2 of the second sensor, whereby the signals x1 of the first sensor produced by the pressure fluctuations are in phase opposition to the signals x2 of the second sensor produced by the pressure fluctuations, whereby a wanted signal yd reproducing the flow is the deviation from a first signal y1 derived from the signal of the first sensor and from a second signal y2 derived from the signal of the second sensor, and whereby the same-phase interfering signals superimposed on the anti-phase sensor signals are eliminated by subtraction. Moreover, the invention also relates to a vortex flowmeter device, which is operated with the above-mentioned method.
Description of Related Art
The measuring principle of vortex flowmeter devices is based on eddies that are generated by baffles arranged in the measuring tube and around which fluid flows. The fluid can be a gas, vapor or a liquid. Strouhal was the first to observe that the eddy generation frequency is proportional to the flow rate of the fluid in the measuring tube, and thus, the eddy-generation frequency is a measure of the flow through the measuring tube expressed in terms of volume flow. Using the density of the fluid, flow can also be indicated as a mass flow rate. The flow field of the fluid produced by the baffle was investigated by Kármán and described mathematically, and thus, the flow field is also referred to as a Kármánic vortex path. The proportional dependency between the flow rate and the eddy generating frequency is described by the Strouhal number that is dependent upon the Reynolds number. The dependency of the Strouhal number on the Reynolds number is considerably influenced by the configuration of the baffle. In current vortex flowmeter devices, the error relative to the volume flow for fluids with a Reynolds number of between 10,000 and 20,000 is less than ±2% and for fluids with a Reynolds number of greater than 20,000 is less than ±1%. Vortex flowmeter devices are distinguished by a mechanically sturdy design and low sensitivity to wear and tear, corrosion and deposits. They can measure gases and vapors as well as liquids with Reynolds numbers over a wide range independently of pressure and temperature with good accuracy and independently of the installation position. Because of the above-mentioned properties, vortex flowmeter devices are used in a number of applications for flow-metering of fluids, in particular aggressive fluids, for example, in the petrochemical, chemical, pharmaceutical or food industry.
Vortex flowmeter devices that are known from the state of the art measure the eddy frequency for pressure fluctuations in the flowing fluid, accompanying the eddies, usually indirectly via the measurement of variations of pressure in the flowing fluid effected by the eddies. Often, the baffles are configured in such a way that the pressure changes exert a force on the baffle and correspondingly deflect or deform the baffle, whereby a first and a second piezoelectric sensor are arranged on the deformation spots. A mechanical excitation of sensors by a deformation caused by a pressure fluctuation produces a change in the polarization of the sensors and thus releases charge carriers in the sensors in such a way that by the mechanical excitation, the first sensor has a positive charge as a signal, and the second sensor has a negative charge as a signal. The sensitivity of a piezoelectric sensor is described by the charge that will develop as a function of the acting forces. In pressure fluctuations caused by eddies, the phase positions are always opposite to the charge signals of the sensors. For either of the two sensors, the signal processing comprises a charge amplifier and a subtractor, whereby the charge amplifiers convert the charge signals into proportional voltage signals and the subtractor subtracts the voltage signals from one another, and the flow is derived from the resulting useful-voltage signal.
By forming the deviation of the signal of the first sensor and the signal of the second sensor, the mechanical interfering excitations that generate interfering signals of the same phase position and the same amplitude in the sensors are eliminated if both sensors have the exact same sensitivity, and the signal processing for both sensors is exactly symmetrical. The mechanical interfering excitations are produced by, for example, turbines, which transfer interfering oscillations to the measuring tube, and in this way, the sensors produce mechanical excitations such that both sensors produce signals of the same phase position and the same amplitude to a very large extent.
Because of manufacturing tolerances—for example, of the sensors themselves or in the design of the sensors—the first sensor and the second sensor have different sensitivities, interfering oscillations of the same phase position and the same amplitude can, however, generate signals—specifically with the same phase position, but different amplitude—in the sensors. In addition, the signal processing, for example, by tolerances of the components used in the signal processing, such as condensers and resistors, is not symmetrical for the signals of the first and second sensors. After the forming of the deviation, an interfering signal remains in the wanted signal and impairs the accuracy of the generic vortex flowmeter devices. Calibration by trimming the sensors is associated with high costs and great expense. Aside from this, the long-term stability of the sensitivity is unknown, so that optionally impractical calibration in the installed state is necessary.